A number of vaccines are currently available or noted in the literature which confer either complete or partial immunity to influenza virus infection by inducing the production of protective antibodies which are active against the viral surface antigens, the proteins hemagglutinin and neuraminidase.
Complete immunity may be understood to be that conferred by conventional vaccines consisting of whole or disrupted virus particles containing those antigens in varying degrees of purification which generate antibodies which block the replication of invading virus particles and thereby prevent the occurrence of disease.
Partial immunity may be understood to be that conferred by vaccines containing those antigens of virus particles, in varying degrees of purification, which allow replication of invading virus particles yet restrict the disease to a substantially asymptomatic process resulting in the establishment of natural immunity.
The stimulation of antibody production, or immunogenicity of the neuraminidase antigen of influenza vaccine virus is considered to be a minor component in the establishment of complete immunity, but the major factor in establishing a state of partial immunity. See J. Infect. Diseases, 140, 844 (1977). Influenza vaccines have recently been prepared which incorporate isolated neuraminidase antigen as the sole active component. See U.S. Pat. Nos 4,029,763 and 4,136,168.
The immunogenicity of the neuraminidase antigen has been found to closely parallel the enzymatic activity of the neuraminidase molecule. The stability of this activity will affect storage life of the vaccine. This is particularly true of influenza virus vaccines of neuraminidase Type 1 which are very labile at 4.degree. C. storage temperatures. Our tests have indicated that such influenza virus vaccines stored at 4.degree. C. lost almost all detectable neuraminidase enzyme activity after approximately 2 to 3 months. This brief storage life has caused obvious problems in the distribution of vaccine and has hampered attempts to calculate the actual effective immunogenic dose. See, D. Bucher and D. Palese, The Biologically Active Protein of Influenza Virus: Neuraminidase, in The Influenza Viruses and Influenza (E. D. Kilbourne, Ed.), Academic Press, N.Y. (1975), at page 93.
Casein hydrolysates (digests) have been employed as one component of vaccine-stabilizing media and have been used to enhance the growth of rabies virus. For example, U.S. Pat. No. 4,147,772 discloses a stabilized vaccine consisting of an activated or attenuated virus, partially hydrolyzed gelatin, a polyhydric alcohol and a buffered acidic medium.
U.S. Pat. No. 3,783,098 discloses stabilization of cell-free Group B Herpes virus suspensions during extraction and lyophilization with skim milk or casein and an amine, which may be an enzymatic digest of milk protein.
U.S. Pat. No. 3,629,399 discloses an orally-administrable polio vaccine comprising an attenuated virus stabilized with phosphate buffer, casein, and casein hydrolysate.
U.S. Pat. No. 3,143,470 discloses a process for the production of a live rabies virus in a medium containing at least 0.3% by weight of pancreatic digest of casein.
It has been discovered that a casein hydrolysate such as N-Z AMINE.RTM. NAK or N-Z AMINE.RTM. A, (Sheffield Products, Memphis, Tennessee) or simpler amino acids will effectively stabilize the neuraminidase activity of stored influenza vaccines, resulting in an enhanced effective period of usefulness of such vaccines.